A conventional hydraulic braking system is disclosed in U.S. Pat. No. 4,878,715 published on Nov. 7, 1989. As shown in FIG. 3 of this patent, the hydraulic braking system includes a wheel brake cylinder and a master cylinder. The wheel brake cylinder is operatively connected to a wheel of a vehicle to apply braking force to the wheel. The master cylinder generates a hydraulic pressure in response to depression of a brake pedal and supplies the hydraulic pressure to the wheel brake cylinder. The master cylinder is connected to the wheel cylinder via a first conduit and a normally open solenoid valve is disposed in the first conduit. The wheel brake cylinder is connected via a second conduit to a reservoir storing brake fluid discharged from the wheel brake cylinder. A normally closed solenoid valve is disposed in the second conduit. The reservoir is connected via a third conduit to a junction portion of the first conduit between the normally open solenoid valve and the master cylinder. A hydraulic pressure pump is disposed in the third conduit. An inlet of the hydraulic pressure pump is connected to the reservoir and an outlet of the hydraulic pressure pump is connected to the junction portion of the first conduit.
A change over valve is disposed in the first conduit between the junction portion and the master cylinder. The change over valve is positionable in a first position in which the master cylinder is connected to the normally open solenoid valve and the inlet of the hydraulic pressure pump is disconnected from the master cylinder, and a second position in which the inlet of the hydraulic pressure pump is connected to the master cylinder and the master cylinder is disconnected from the normally open solenoid valve. When the change over valve is positioned in the second position, the hydraulic pressure pump draws brake fluid from a master reservoir via the master cylinder and pumps the brake fluid to the wheel brake cylinder so that hydraulic pressure can be supplied to the wheel brake cylinder even if the brake pedal is not depressed.
However, since this hydraulic braking system includes the change over valve for supplying hydraulic pressure to the wheel brake cylinder when the brake pedal is not depressed, the system is expensive and rather large.